Circuit arrangement for automatic control of the voltage of an electrical filter

ABSTRACT

A control unit comprises a charging circuit which connects a guide capacitor to a source of DC voltage whereby a control voltage supplied to a control system is produced by the control unit in accordance with the voltage of a guide capacitor. A discharging circuit is connected in parallel with the guide capacitor and includes a controllable resistor having a resistance value dependent upon the voltage of the filter. A photoelement is in operative proximity with a glow lamp in a manner whereby light produced by the glow lamp impinges on the photoelement. The photoelement is coupled by a circuit to the discharging circuit for supplying a control signal dependent upon the irradiation of the photoelement whereby the control signal controls the controllable resistor of the discharging circuit to a low resistance value when the glow lamp does not glow and controls the controllable resistor to a high resistance value when the glow lamp glows. A cutoff circuit supplies a cutoff signal to the control system and the control system controls a control circuit to interrupt the filter current without delay when a sparkover occurs in the filter.

United States Patent Vukasovic et al.

CIRCUIT ARRANGEMENT FOR AUTOMATIC CONTROL OF THE VOLTAGE OF AN ELECTRICAL I FILTER Inventors: Lovro Vukasovic; Rudolf l-Ioimann, both Appl. No.: 17,176

[ 1 Feb. 22, 1972 FOREIGN PATENTS OR APPLICATIONS Primary Examiner-Dennis E. Talbert, Jr.

Attorney-Curt M. Avery, Arthur E. Wilfond, Herbert L. Lerner and Daniel J. Tick [5 7] ABSTRACT A control unit comprises a charging circuit which connects a guide capacitor to a source of DC voltage whereby a control voltage supplied to a control system is produced by the control [30] Foreign Application Priority Data unit in accordance with the voltage of a guide capacitor. A discharging circuit is connected in parallel with the guide 3 germany capacitor and includes a controllable resistor having a remany sistance value dependent upon the voltage of the filter. A photoelement is in operative proximity with a glow lamp in a 2% 55/ l manner whereby light produced by the glow lamp impinges on d 'f the photoelement. The photoelement is coupled by a circuit to e 0 are 5 I the discharging circuit for supplying a control signal dependent upon the irradiation of the photoelement whereby the control signal controls the controllable resistor of the [56] References Cited discharging circuit to a low resistance value when the glow UNITED STATES PATENTS lamp does not glow and controls the controllable resistor to a hlgh resistance value when the glow lamp glows. A cutoff Cl!- et al. t cuit up lies a cute ignal to the control system and the con. 3,433,675 V1970 ElShOld "55/2 trol system controls a control circuit to interrupt the filter curi 3 F rent without delay when a sparkover occurs in the filter. u|sse 3,577,708 5/ l 971 Drenning et a1. ..55/ 105 11 Claims, 2 Drawing Figures CURRENT CUNTRUL mmsrunmsn cmcun, HINB u m 1 m m #lRANSFURMER /CONTRUL M A SYSTE REBTIFIER i N FILTER CONTROL k F umr t GLOW LAMP LA PATENTEHFEB22 1912 CURRENT comm TRANSFORMER cmcum M I I EI THINB I TRANSFORMER -w s 11 4 /|IONTRUL AM svsr /mzcnnsn un-a: 1P2:

PHUTUELEMENT CIRCUIT ARRANGEMENT FOR AUTOMATIC CONTROL OF THE VOLTAGE OF AN ELECTRICAL FILTER DESCRIPTION OF THE INVENTION The invention relates to the control of the voltage of an electrical filter, such as an electrical precipitator. More particularly, the invention relates to a circuit arrangement for the automatic control of the voltage of an electrical filter.

A control circuit controls the filter voltage. A control system supplies a control magnitude to the control circuit, which magnitude varies in accordance with a control voltage supplied to a control unit, which control unit is in turn connected to the control system. The control unit includes a guide capacitor connected to a source of DC voltage via a charging circuit. The control voltage depends upon the voltage of the guide capacitor. A discharge circuit is connected in parallel with the guide capacitor and contains a controllable resistor having a resistance value which depends upon the filter voltage.

The control circuit may comprise a control transformer, a transducer, a positioner including thyristors, or a controlled rectifier. The source of energizing voltage may be single or multiphase.

Known circuit arrangements of the aforedescribed type have essentially not been utilized, since the galvanic connection between the filter, conducting high voltage, and the control circuit, resulted in difficulties. These difficulties may be avoided by our invention.

Tests have shown that after a sparkover in the filter, a considerably higher current flows temporarily through the filter. This results in higher maintenance costs and causes the filter to have a shorter lifespan. The temporarily increased filter current is due to the control circuit and control system having time constants, which time constants prevent the filter voltage from being decreased without delay after the occurrence of a sparkover in the filter.

The principal object of the invention is to provide a new and improved circuit arrangement for the automatic control of the voltage of an electrical filter.

An object of the invention is to provide a circuit arrangement for the automatic control of the voltage of an electrical filter which overcomes the difficulties ensuing in similar circuit arrangements previously known.

An object of the invention is to provide a circuit arrangement for the automatic control of the voltage of an electrical filter, which circuit arrangement has lower maintenance costs and results in a longer lifespan for the filter.

An object of the invention is to provide a circuit arrangement for the automatic control of the voltage of an electrical filter, which circuit arrangement prevents the temporary occurrence of an increased current magnitude after sparkover in the filter.

An object of the invention is to provide a circuit arrangement for the automatic control of the voltage of an electrical filter, which circuit arrangement interrupts the filter current by a control circuit whenever a sparkover occurs in the filter.

An object of the invention is to provide a circuit arrangement for the automatic control of the voltage of an electrical filter, which circuit arrangement includes a control system for controlling a control circuit having thyristors by cutting off the supply of control pulses to the thyristors when a sparkover occurs in the filter.

The termination of the cutoff of control pulses, or the reinstallment of said control pulses would result in an abrupt increase in the filter voltage and thereby cause an overshooting of said filter voltage. Another object of the invention is therefore to provide a circuit arrangement for the automatic control of the voltage of an electrical filter, which circuit arrangement prevents overshooting of the filter voltage.

I An object of the invention is to provide a circuit arrangement for the automatic control of the voltage of an electrical filter with efficiency, effectiveness and reliability.

LII

In accordance with the present invention, a circuit arrangement for automatic control of the voltage of an electrical filter comprises a control circuit coupled to an electrical filter for changing the voltage of the filter. A source of voltage is coupled to the control circuit. A control system has an input coupled to the source of voltage, input means and an output connected to the control circuit for supplying to the control circuit a control magnitude. The control magnitude depends upon a control voltage supplied to the input means of the control system. A voltage divider connects a glow lamp in parallel with the filter. A control unit has an input coupled to the source of voltage and output means coupled to the input means of the control system. The control unit comprises a source of DC voltage. A charging circuit connects a guide capacitor to the source of DC voltage whereby the control voltage supplied to the control system is produced by the control unit in accordance with the voltage of the guide capacitor. A discharging circuit is connected in parallel with the guide capacitor and includes a controllable resistor having a resistance value dependent upon the voltage of the filter. A photoelement has a photosensitive area provided in operative proximity with the glow lamp in a manner whereby light produced by the glow lamp impinges upon the photosensitive area of the photoelement. Circuit means couples the photoelement to the discharging circuit for supplying a control signal dependent upon the irradiation of the photosensitive area of the photoelement whereby the control signal controls the controllable resistor of the discharging circuit to a low resistance value when the glow lamp fails to glow and controls the controllable resistor to a high resistance value when the glow lamp glows.

The control unit further comprises a reversing switch. A switching component has a switching condition which depends upon the switching position of the reversing switch and the magnitude of current flowing through the filter. The switching component is in its conductive condition only during the time between the energization of the circuit arrangement, with the assistance of the reversing switch, and the time during which the current flowing through the filter reaches a critical value for the first time after the energization of the circuit arrangement by the reversing switch. The switching component is connected in a manner whereby the control signal is ineffective when the switching component is in its nonconductive condition.

The control unit further comprises auxiliary circuit means connecting the switching component in parallel to the charging circuit of the guide capacitor.

The control unit further comprises a capacitor. A monostable multivibrator has a pair of input and output transistors of the same conductivity type and the controllable resistor of the control unit comprises a first transistor having a base-emitter path coupled via the capacitor and a resistor to the output of the monostable multivibrator whereby when the glow lamp glows, the input transistor of the monostable multivibrator is in its fully conductive condition. The output of the monostable multivibrator is connected to the negative polarity terminal of the source of DC voltage via a resistor, a decoupling diode and the emitter-collector path of a second transistor. The switching component comprises the second transistor whereby when the second transistor is in its fully conductive condition approximately the same potential is applied at the output of the monostable multivibrator as at the extinguished glow lamp, a third transistor having a control path connected in parallel with the first transistor and a base electrode. A diode connects the base electrode of the third transistor to a reversible voltage divider and conducts the control current of the third transistor. The control current of the third transistor flows through the reversible voltage divider as long as the current flowing through the filter is below the critical value and the first transistor is in a condition other than its fully conductive condition.

The control unit further comprises a fourth transistor of the same conductivity type as the pair of transistors of the lOl024 0088 monostable multivibrator. A relay has an energizing winding connected to the collector electrode of the fourth transistor. A control circuit for the relay winding includes the base-emitter path of the fourth transistor. The base-emitter path is connected to the relay winding. A decoupling diode connects the "control circuit of the relay in parallel with the output of the monostable multivibrator in a manner whereby when the glow lamp glows and the second transistor is in its nonconductive condition a control current flows. The fourth transistor has an emitter electrode connected to the collector electrode of the second transistor. The emitter electrode of the fourth transistor has a potential of a magnitude which is such that when the second transistor is in its conductive condition the fourth transistor remains in its fully conductive condition.

In accordance with the invention, a circuit arrangement for automatic control of the voltage of an electrical filter comprises a control circuit coupled to an electrical filter for changing the voltage of the filter. A source of voltage is coupled to the control circuit. A control system has an input coupled to the source of voltage, input means and an output connected to the control circuit for supplying to the control circuit a control magnitude depending upon a control voltage supplied to the input means of the control system. A control unit has an input coupled to the source of voltage and output means coupled to the input means of the control system. The control unit comprises a source of DC voltage. A charging circuit connects a guide capacitor to the source of DC voltage whereby the control voltage supplied to the control system is The capacitor and the discharging circuit have time constants which prevent the termination of the fully conductive condition of the second transistor prior to the termination of the produced by the control unit in accordance with the voltage of the guide capacitor. A discharging circuit is connected in parallel with the guide capacitor and includes a controllable resistor having a resistance value dependent upon the voltage of the filter. Cutoff circuit means supplies a cutoff signal to the input means of the control system. The control system controls the control circuit for interrupting the filter current without delay when a sparkover occurs in the filter.

The control circuit comprises thyristor means and the control magnitude supplied by the control system controls the thyristor means. The cutoff circuit means of the control unit switches the thyristor means of the control circuit via the control system when a sparkover occurs in the filter.

The cutoff circuit means of the control unit comprises a first transistor having an emitter-collector path and a control path. The cutoff signal is derived from a point in the emitter-collector path of the first transistor. A second transistor of complementary type to the first transistor has a control path connected in parallel with the control path of the first transistor. The controllable resistor of the discharging circuit comprises the control path of the second transistor. Each of the first and second transistors has a base electrode connected to a first voltage divider. The first transistor is in its fully conductive condition during normal operation of the filter. A third transistor of the same conductivity type as the first transistor is in its nonconductive condition during normal operation of the filter and switches to its conductive condition upon the occurrence of a sparkover in the filter. The third transistor has an emitter electrode coupled to the emitter electrode of the second transistor. The third transistor has a collector electrode coupled to the base electrode of the second transistor via a capacitor and a resistor. The collector electrode of the third transistor is connected to the tap point of the first voltage divider. The capacitor discharges via the control path of the second transistor when the third transistor is in its conductive condition whereby the discharging of the capacitor switches the second capacitor to its conductive condition. A second voltage divider is connected to the base electrode of the second transistor via a diode. The diode is connected with a polarity which is such that the second voltage divider is supplied a fully advanced control current via a reversing switch in an operating position of the reversing switch and said second transistor is in its nonconductive condition.

The control unit further comprises a fourth transistor funcreset condition of the monostable multivibrator.

The control unit further comprises a diode connecting the second voltage divider to the reversing switch. -A fifth transistor has an emitter-collector path connected to the second voltage divider via the diode. The reversing switch has an operable switching position wherein it connects the second voltage divider to the source of DC voltage and wherein the circuit arrangement further comprises a current transformer connected between the source of voltage and the control circuit and has an output connected to the input of the control unit for supplying a signal to the control unit when the current flowing through the filter exceeds a predetermined critical value. The fifth transistor of the control unit is fully controlled by the signal supplied by the current transformer to the control unit.

The control unit further comprises a resistor. A second guide capacitor is connected to the first-mentioned guide capacitor via the resistor. The control voltage is derived from the second guide capacitor. The controllable resistor of the discharging circuit is connected in parallel with the second guide capacitor via another resistor. The period of interruption of the filter current and the time constant of the second guide capacitor and the discharging circuit are adjusted to each other in a manner whereby upon the termination of the interruption the second guide capacitor is substantially completely discharged. The second guide capacitor and the resistor have a time constant which is such that the voltage of the filter increases a s rapidly as possible without overshootmg.

It is frequently preferred that a signal be produced when the glow lamp remains extinguished for longer than a specific period of time, since this usually indicates a metallic short circuit in the filter. The relay is utilized for this purpose. The relay is coupled to the source of DC voltage via the emittercollector path of a transistor, the control path of which receives the control signal. The relay is thus unable to produce a faulty alarm signal when the switching component is in its fully conductive condition, that is, during the acceleration period. The emitter electrode of the transistor may also be coupled to the switching component via a resistor and a diode, so that when the resistor has a suitable resistance value, the transistor will remain in its fully conductive condition even when the switching component is in its nonconductive conditron.

In order that the invention may be readily carried into effect, it will now be described with reference to the accompanying drawing, wherein:

FIG. I is a block diagram of an embodiment of the circuit arrangement of the invention; and

FIG. 2 is a circuit diagram of the control unit Sr of FIG. 1.

In FIG. 1, an electrical filter F is coupled to the secondary winding of a high voltage transformer T via a rectifier G. The electrical filter, also called electrical precipitator, is shown diagrammatically as a high voltage wire d and a grounded metallic tube t. The primary winding of the transformer T is coupled to an input terminal U of a single or multiphase AC voltage power system via a smoothing choke D, a control circuit S and a current transformer W. The current transformer W, the control circuit S and the smoothing choke D are connected in series circuit arrangement between the input terminal U and the transformer T. The smoothing choke D functions to improve the shape of the signals supplied to the transformer T. The control circuit S comprises thyristors connected in antiparallel arrangement, that is, with the anode of one connected to the cathode of the other and the cathode of the first connected to the'anode of the second.

Control pulses for the thyristors of the control circuit S are supplied by a control system I. In order to synchronize the control system I, the alternating voltage of the AC voltage tioning with the third transistor as a monostable multivibrator. power supply, applied via the input terminal U, is applied to said control system. A control voltage U is applied to the control system I via an input terminal A. The control voltage U determines the phase position of the control pulses relative to the alternating voltage of the AC voltage power supply. Control system I can consist of a known standard unit, for instance control unit teb-p4 se 401, manufactured by Siemens AG, Germany.

The provision of control pulses by the control system I may be blocked without delay, and without regard to the mag-- nitude of the control voltage U by a cutoff signal supplied to said control system via an input terminal L thereof. The control voltage U and the cutoff signal are supplied by a control unit St. The cutoff signal is supplied to the control system I by the control unit St when a short circuit or similar fault occurs inthe filter F.

At the onset of operations, a specific instance may occur. The control voltage U is applied to the control system I by the control unit St and depends upon a signal supplied to said control unit by the current transformer W. The signal supplied by the current transformer W to the control unit St is proportional to the filter current and indicates a short circuit, breakdown, light arc, or the like, in the filter F. The signal is obtained via a photosensitive semiconductor device included in the control unit St and exposed to light produced by a glow lamp LA. The glow lamp LA is connected in parallel with the filter F via a pair of resistors r28 and r29, so that said glow lamp glows only when said filter is in operation. When a sparkover occurs in the filter F, the glow lamp LA is extinguished.

FIG. 2 illustrates the circuit arrangement of the control unit St. The control unit St includes a rectifier g, which is preferably a full-wave rectifier, and has applied to it via an input terminal B a voltage proportional to the filter current. The rectifier g is connected via a resistor r27 to an RC member 06, r26. The resistor r26 of the RC member has a variable resistance. The variable resistance of the resistor r26 is coupled in parallel with voltage divider r24, r25 via a Zener diode n14. The resistor r24 of the voltage divider r24, r25 is connected in parallel with a series circuit arrangement of the control path of a transistor p and a thyristor p9. The collector electrode of the transistor p10 is coupled to a positive polarity terminal P of a DC voltage source via a resistor r6 and a resistor r8. The emitter electrode of the transistor p10 is connected to the control electrode of the thyristor p9. The base electrode of the transistor p10 is connected to a common point in the connection between the resistors r24 and r25 of the voltage divider r24, 425. I

The direct voltage source has, in addition to the positive polarity terminal P, a negative polarity terminal N and an intermediate tenninal or tap M, and delivers a voltage of 24 volts between said positive polarity terminal and said intermediate terminal and between said intermediate terminal and said negative polarity terminal. A series circuit arrangement of a first guide capacitor 04, a resistor r20, a diode n10, the emitter-collector path of a a transistor p8 and a diode n13 is connected between the intermediate terminal M and the negative polarity terminal N. The base electrode of the transistor p8 is connected to the anode of the thyristor p9. The cathode of the thyristor p9 is connected to the negative polarity terminal N.

The emitter-collector path of a transistor p7 is connected in parallel with a resistor r20, a'diode n10, the transistor p8 and the diode n13, all of which components are connected in series circuit arrangement. The emitter-collector path of the transistor p7 is connected in series circuit arrangement with a resistor r18. The base electrode of the transistor p7 is connected to a voltage divider r16, r17 which is connected between the intermediate terminal M and the emitter electrode of the transistor p8. The transistor p7 maintains constant a charging current flowing therethrough. The transistor p7 maintains the charging current constant at a value which may be readily adjusted, within a large range, by the variable resistor rl7 of the voltage divider r16, r17, or by the resistor r18.

The first guide capacitor 04 is connected in parallel with a second guide capacitor 05 via a resistor r15. The control voltage U,,,,, is derived from the second guide capacitor CE for the control system I. The capacitance of the second guide capacitor 05, however, is only a fraction of the capacitance of the first guide capacitor c4.

In order to discharge the first guide capacitor (:4, in accordance with the filter current, said capacitor is connected in parallel with the emitter-collector path of a transistor p4 via a resistor 11. The base electrode of the transistor p4 is connected to the positive polarity terminal P via a resistor r8.

To discharge the guide capacitors during a sparkover in the filter F, the emitter-collector path of a transistor p5 is connected in parallel with the first guide capacitor c4 via a variable resistor r13 and is connected in parallel with the second guide capacitor c5 via a resistor r14 and a diode n9. The time constant provided by the second guide capacitor c5 and the resistor r14 is selected so small that said capacitor is discharged when the transistor p5 is in fully conductive condition whereby the discharge is substantially complete due to a sparkover in the filter F. On the other hand, the charging time constant determined by the second guide capacitor 05 and the resistor r15 is also so small that the voltage at said capacitor increases extremely rapidly to the voltage of the first guide capacitor 04 after the elimination of a sparkover in the filter F and the switching of the transistor p5 to its nonconductive condition.

The discharge of the first guide capacitor 04 during a sparkover of the filter F and during fully conductive condition of the transistor p5, is much slower and is determined by the resistance value of the variable resistor r13. Thus, after a sparkover in the filter F, the voltage at the first guide capacitor 04 is lower than that prior to the sparkover by only a small percentage adjustable by the variable resistor rl 3.

A sparkover of the filter F is measured by the glow lamp LA. The glow lamp LA is positioned above a photoelement f in a manner whereby the photosensitive region of said photoelement is exposed to light produced by said glow lamp.

The photoelement f is connected in parallel with the baseemitter path of a transistor pl via a diode nl. The transistor pl and the diode n1 are so connected that during the irradiation of the photoelement f, the voltage at said photoelement controls said transistor to its fully conductive condition.

The transistor pl and a transistor p2 function as a monostable flip-flop with resistors r2, r3 and r4 and a capacitor cl. When the glow lamp LA glows or produces light, that is, during normal operation of the filter, the transistor p1 is in its fully conductive condition and the transistor p2 is in its nonconductive condition. When there is a sparkover in the filter F, and during the occurrence of such sparkover, the glow lamp LA is extinguished or quenched and the transistor pl is switched to its conductive condition. The transistor p2 is then switched to its fully conductive condition. The capacitor cl, which was charged via the resistors r4 and r2 and the transistor pl, discharges via the transistor p2, a diode n3, the diode n2 and the resistor r2 thereby maintaining, regardless of the operational condition of the filter F, the transistor pl in its nonconductive condition. The full control of the transistor p2 is thereby maintained, regardless of the operational condition of the filter F for a specific period determined by the time constant of the discharge circuit.

The base electrode of the transistor p5 is connected to the collector electrode of the transistor p2 via a resistor. r9 and a capacitor 03 connected in series circuit arrangement with each other. The base electrode of the transistor p5 is connected to the negative polarity terminal N via a resistor r19. A resistor r10 is connected in parallel with the series circuit arrangement of the resistor r9 and the capacitor c3. When the transistor p2 is in its nonconductive condition, during normal operation of the filter F, the capacitor c3 is charged to a voltage having the indicated polarity. When the transistor p2 is in its fully conductive condition, the capacitor c3 may discharge via the transistor p2, a diode n3, the emitter-base path of the transistor p and the resistor r9. The transistor p5 is thus always in its fully conductive condition simultaneously with the transistor p2. The discharge circuit of the capacitor c3 is preferably so rated that the fully conductive condition of the transistor p5 is not terminated prior to the termination of the flip-flop reset time of the monostable flip-flop circuit.

Each of the transistors p1, p2, p3, p7, p8 and p is an NPN- type transistor. Each of the transistors p4 and p5 is a PNP-type transistor. A transistor p6 is an NPN-type transistor. The control path of the transistor p5 is connected in parallel with the control path of the transistor p6. The emitter-collector path of the transistor p6 is connected between the positive polarity terminal P and the intermediate terminal M via a resistor r12. The output tenninal L of the control unit St, connected to the control system I, is connected to the collector electrode of the transistor p6.

A voltage divider r4, r10, r19 is so rated that the voltage applied to the control path of the transistor p6 when the transistor p2 is in its nonconductive condition, is sufficient to switch the transistor p6 to its fully conductive condition. The transistor p5 is then in its nonconductive condition. The base electrode of the transistor p6 is coupled to the positive polarity terminal P via a diode n8 and the resistor r8. The resistance values of the resistors r8 and r6 are so rated that the potential at the junction point of said resistors is such that the diode n8 is in its nonconductive condition. This occurs during the illustrated position of a reversing switch s shown in FIG. 2, or when the transistor p10 is in its fully conductive condition. The diode n8 is in its conductive condition when the reversing switch s is in its position not shown in FIG. 2 and the transistor p10 is in its nonconductive condition.

In order to determine a metallic short circuit in the filter F, the control unit St includes a relay R having a working contact. The relay R is coupled between the positive and negative polarity terminals P and N via the emitter-collector path of a transistor p3, a diode n5 and the reversing switch s. The base electrode of the transistor p3 is connected to the' collector electrode of the transistor p2 via resistors r5 and r7 and a diode n7. The transistor p3 is thus switched to its fully conductive condition when the transistor p2 is switched to its conductive condition, during normal operation of the filter F.

A capacitor 02 is connected between a common point in the connection between the resistors r5 and r7 and the emitter electrode of the transistor p3. The capacitor 02 has a relatively high capacitance of a magnitude which determines, together with the resistance value of the resistor r5, the time which elapses between a short circuit in the filter F, the transistor p2 being in its fully conductive condition, and the deenergization of the relay R.

The collector electrode of the transistor p8 is connected to the collector electrode of the transistor p2 via a diode nll and a resistor r21 connected in series circuit arrangement. The collector electrode of the transistor p8 is also connected to the emitter electrode of the transistor p3 via a diode n12 and a resistor r22. The emitter electrode of the transistor p3 is connected to the base electrode of the transistor p8 via a resistor 123. The resistor r21 is rated at approximately the same resistance value as the resistor r4, so that when the transistor p2 is in its nonconductive condition, and the transistor p8 is in its fully conductive condition, the approximate potential of the intermediate terminal is applied to the collector electrode of the transistor p2. This prevents the capacitor c3 from being charged while the transistor p8 is in its fully conductive condition, during the increasing of the voltage at the filter F, following the switching of said transistor to its conductive condition. The periodic extinguishing of the glow lamp LA, when the transistor p2 is in its fully conductive condition, due to the ripple or pulsation factor of the filter voltage during the increase thereof, may therefore not result in a voltage drop during such period. A control current is therefore supplied to the transistor p6 via the resistor r8 and the diode n8, if the reversing switch s is in its condition opposite that shown in FIG. 2 and the transistor p10 is in its nonconductive condition.

The resistance values of the relay R and of the resistor r22 are so rated that the emitter electrode of the transistor p3 is sufficiently negative during such condition of operation that the diode n5 is switched to its nonconductive condition and the transistor p3 remains in its fully conductive condition.

The reversing switch s functions to maintain a predetermined initial condition during the commencement of the installation. In the position of the reversing switch s shown in F IG. 2, the base electrode of the transistor p8 is connected to the negative polarity terminal N via the resistor r23 and the diode n5, so that said transistor is switched to its nonconductive condition. On the other hand, the transistor p4, which is connected in parallel with the first guide capacitor c4, is in its fully conductive condition, since the base electrode of said transistor is connected to the negative polarity terminal N via the resistor r6 and the diode n6. Since no voltage is applied to the filter F, the transistors p2 and p5 are in their fully conductive condition. The transistor p6 is in its nonconductive condition and a cutoff signal is supplied to the terminal L of the control unit St. The first and second guide capacitors c4 and 05 are discharged.

When the reversing switch s is in its position shown in FIG. 2, the thyristor p9 is quenched, extinguished or switched to its nonconductive condition, after the installation is switched off.

To initiate the installation the reversing switch s is moved into its second switching position, opposite to that shown in FIG. 2, in which the cathodes of the diodes n5 and n6 are connected to the intermediate tenninal M. The junction point of the resistors r6 and r8 thus becomes so positive that the transistor p4 is switched to its nonconductive condition and the transistor p6 is switched to its conductive condition. This eliminates the cutoff signal supplied to the terminal L of the control unit St, so that the control system I (FIG. 1) supplies control pulses to the control circuit 5 (FIG. 1). The phase position of the control pulses supplied to the control circuit S by the control system I depends upon the control voltage U M at the second guide capacitor c5.

The transistor p8 is supplied with a control current which switches it to its fully conductive condition, via the transistor p3 and the resistor r23. The first and second guide capacitors c4 and 05 are therefore charged to a voltage of the polarity in dicated' in FIG. 2, via the resistor r20, the diode n10, the transistor p8 and the diode n13. The charging time constant has a very low value, since the filter voltage, which depends upon the voltage at the second guide capacitor c5, is to obtain the breakthrough value as soon as possible, following the switching on of the installation.

As soon as the transistor p8 is in its conductive condition, after the reversal of the reversing switch s, a current also flows via the resistors r4 and r2] and the diodes n11 and n13. At the aforedescribed rating of the resistors, this means that the potential at the collector electrode of the transistor p2 corresponds approximately to the potential of the intermediate terminal M, regardless of the operating condition of the filter F and the condition of the monostable flip-flop or multivibrator circuit. Despite this, the transistor p3 is supplied with a control current via the diode n7, the resistor r7, the resistor r5, the resistor r22, the diode n12, the emitter-collector path of the transistor p8 and the diode n13, since the emitter potential of the transistor p3 is more negative at the aforedescribed rating of the relay R and the resistor r22 than the potential of the intermediate terminal M. The relay therefore remains in its energized condition.

During the rapid charging of the first and second guide capacitors c4 and 05 via the transistor p8 in its conductive condition, the transistor p6 is supplied a full control current, via the resistor r8 and the diode n8. Thus, no cutoff signal is supplied to the terminal L of the control unit St. The control system I (FIG. 1) therefore delivers control pulses to the control circuit S. The phase position of the control circuit therefore depends upon the control voltage U, at the second guide capacitor 05.

The increase of the voltage at the second guide capacitor c and the corresponding increase of the filter voltage causes the filter current to increase also. The voltage derived at the resistor r26 thus also increases. At a specific value of the voltage at the resistor r26, which is the reference value of the filter current, the Zener diode n14 is switched to its conductive condition and conducts current and the transistor p and the thyristor p9 are switched to their fully conductive condition. Thence the current through the relay R flows through the thyristor p9, via the transistor p3 and the resistor r23. The transistor p8 is switched to its nonconductive condition, since the control path of said transistor is short-circuited by the transistor p9. This terminates the exponential rapid charging of the first and second guide capacitors via the transistor p8 and the output signal provided by the monostable multivibrator via the resistor r21 and the diode n11.

The thyristor p9 remains in its conductive condition until the reversing switch s is reversed in position or the installation is switched off. Regardless of the conductive condition of the transistor p10, the thyristor p9 remains in its fully conductive condition. The first and second guide capacitors c4 and c5 may therefore be charged with a constant current during operation, only via the transistor p7 and the resistor r18. The charge of the guide capacitors c4 and c5 is substantially independent of the ambient temperature, due to the diode n13 coupled to the base electrode or control circuit of the transistor p7. This is of considerable importance when the time constant is very large. The control circuit of the transistor p7 is of relatively low resistance, so that it is easily possible to position the variable resistor r17 far away from the remainder of the installation, on a control board.

If the filter current exceeds the reference value during operation, the transistor p10 and the transistor p4 are temporarily in their fully conductive condition, so that the first and second guide capacitors c4 and 05 are slightly discharged. The filter voltage therefore decreases. The filter current again decreases to a value less than the reference value and the transistors p10 and p4 are switched to their nonconductive condition. The voltage of the first and second guide capacitors c4 and c5 and of the filter F thereafter slowly commences to increase linearly. The filter current is limited in this manner to the reference value, without a voltage-dependent decrease, during the time that no sparkover occurs in the filter F.

During a sparkover in the filter F, the transistor p5 is switched to its fully conductive condition by the discharge of the capacitor (:3, regardless of the magnitude of the filter current. The first guide capacitor 04 is thereby discharged, via the resistor r13, by a specific, relatively small, amount. The second guide capacitor c5 discharges almost completely, via the resistor r14 and the diode n9. Furthermore, when the transistor p5 is in its fully conductive condition, the transistor p6 is switched to its nonconductive condition, so that a control pulse is not supplied to the control circuit S (FIG. 1) due to the supply of a cutoff signal to the terminal L of the control unit St.

Upon the termination of the reset time of the monostable flip-flop circuit, the transistor p5 is switched to its nonconductive condition and the second guide capacitor c5 is charged relatively rapidly to the voltage of the first guide capacitor 04. The filter voltage increases correspondingly rapidly, since the transistor p5 is in its conductive condition. Therefore, there is no longer a cutoff signal at the terminal L. The rate of charging of the second guide capacitor c5 is adjusted to the installation in a manner whereby the filter voltage increases as rapidly as possible without varying abruptly the value determined by the first guide capacitor 04.

During the exponential charging of the first and second guide capacitors c4 and 05, with the transistor p8 in its fully conductive condition, the transistor p5 may not be supplied with a control current via the resistor r18. This is due to the fact that the transistor p6 is supplied with a control current for switching it to its fully conductive condition, via the resistor r8 and the diode n8, so that the voltage applied to the base emitter path of the transistor p6 is applied as a blocking bias voltage to the control path of the transistor p5. The control current for the transistor p6, via the resistor r8 and the diode n8, is omitted, however, if the filter current reaches the value of the reference current at the end of the voltage increasing period and the transistor p10 is thereby fully controlled or switched to its fully conductive condition. If there is no sparkover in the filter F at such time, and the transistor p2 of the monostable flip-flop circuit is therefore in its nonconductive condition, the transistor p6 is supplied with a full control current via the resistors r4 and r10 and is switched thereby to its fully conductive condition.

If, however, during the initial response of the transistor p10 after the initiation of the installation a sparkover or a light arc occurs in the filter F, as a result of which the transistor p2 is switched to its fully conductive condition, the transistor p6 is not supplied with control current, either via the resistors r4 and r10 or the resistor r8 and the diode n8, so that the transistor p6 is switched to its nonconductive condition and a cutoff signal is provided at the output terminal L for obstructing the control pulse provided for the control circuit F.

Simultaneously, the transistor p5 is supplied with a full control current via the resistor r19 to switch said transistor to its fully conductive condition. This assures the discharge of the second guide capacitor c5. A full control of the transistor p5 by the capacitor 03, via the transistor p2, would not be possible, however, at the termination of the exponential charging of the guide capacitors, since the capacitor c3 could not yet be charged due to the transistor p8 being in its full conductive condition. Thus, although in the period following the exponential charging of the guide capacitors, a cutoff signal is provided at the output terminal L and the discharge of both said guide capacitors is effected by the transistor p5 during each sparkover of the filter F, regardless of the filter current, a voltage drop in said transistor and the provision of a cutoff signal are initiated after the exponential charging, if a sparkover or a light arc is present in the filter and if the filter current has reached or exceeded the reference value.

The invention may also be utilized in installations for the control of an electron beam or an ion beam, for material processing.

While the invention has been described by means of a specific example and in a specific embodiment, we do not wish to be limited thereto, for obvious modification will occur to those skilled in the art without departing from the spirit and scope of the invention.

We claim:

1. A circuit arrangement for automatic control of the voltage of an electrical filter, said circuit arrangement comprising a control circuit coupled to an electrical filter for changing the voltage of said filter;

a source of voltage coupled to said control circuit;

a control system having an input coupled to said source of voltage, input means and an output connected to said control circuit for supplying to said control circuit a control magnitude, said control magnitude depending upon a control voltage supplied to the input means of said control system;

a glow lamp;

a voltage divider connecting said glow lamp in parallel with said filter; and

a control unit having an input coupled to said source of voltage and output means coupled t the input means of said control system, said control unit comprising a source of DC voltage, a guide capacitor, a charging circuit connecting said guide capacitor to said source of DC voltage whereby the control voltage supplied to said control system is produced by said control unit in accordance with voltage of said guide capacitor, a discharging circuit connected in parallel with said guide capacitor and including a controllable resistor having a resistance value dependent upon the voltage of said filter, a photoelement having a photosensitive area provided in operative proximity with said glow lamp in a manner whereby light produced by said glow lamp impinges upon the photosensitive area of said photoelement, and circuit means coupling said photoelement to said discharging circuit for supplying a control signal dependent upon the irradiation of the photosensitive area of said photoelement whereby said control signal controls the controllable resistor of said discharging circuit to a low resistance value when said glow lamp fails to glow and controls said controllable resistor to a high resistance value when said glow lamp glows.

2. A circuit arrangement as claimed in claim 1, wherein said control unit further comprises a reversing switch, a switching component having a switching condition which depends upon the switching position of said reversing switch and the magnitude of current flowing through said filter, said switching component being in its conductive condition only during the time between the energization of said circuit arrangement, with the assistance of said reversing switch, and the time during which the current flowing through said filter reaches a critical value for the first time after the energization of said circuit arrangement by said reversing switch, said switching component being connected in a manner whereby said control signal is ineffective when said switching component is in its nonconductive condition.

3. A circuit arrangement as claimed in claim 2, wherein said control unit further comprises auxiliary circuit means connecting said switching component in parallel to the charging circuit of said guide capacitor.

4. A circuit arrangement as claimed in claim 2, wherein said control unit further comprises a capacitor, a resistor, a monostable multivibrator having a pair of input and output transistors of the same conductivity type and the controllable resistor of said control unit comprises a first transistor having a base-emitter path coupled via said capacitor and said resistor to the output of said monostable multivibrator whereby when said glow lamp glows, the input transistor of said monostable multivibrator is in its fully conductive condition, a resistor, a decoupling diode, a second transistor, the output of said monostable multivibrator being connected to the negative polarity terminal of said source of DC voltage via said resistor, decoupling diode and the emitter-collector path of said second transistor, said switching component comprising said second transistor whereby when said second transistor is in its fully conductive condition approximately the same potential is applied'at the output of said monostable multivibrator as at the extinguished glow lamp, a third transistor having a control path connected in parallel with said first transistor and a base electrode of the third transistor, a reversible voltage divider, a diode connects the base electrode to said reversible voltage divider and conducting the control current of said third transistor, the control current of said third transistor flowing through said reversible voltage divider as long as the current flowing through said filter is below the critical value and said first transistor is in a condition other than its fully conductive condition.

5. A circuit arrangement as claimed in claim 4, wherein said control unit further comprises a fourth transistor of the same conductivity type as the pair of transistors of the monostable multivibrator, said fourth transistor having a collector electrode, a relay having an energizing winding connected to the collector electrode of said fourth transistor, a control circuit for said relay winding including the base-emitter path of said fourth transistor, said base-emitter path being connected to said relay winding, a decoupling diode connecting the control circuit of said relay in parallel with the output of said monostable multivibrator in a manner whereby when the glow lamp glows and said second transistor is in its nonconductive condition a control current flows, a resistor and a decoupling diode, said fourth transistor having an emitter electrode connected to the collector electrode of said second transistor, the emitter electrode of said fourth transistor having a potential of a magnitude which is such that when said second transistor is in its conductive condition said fourth transistor remains in its fully conductive condition.

6. A circuit arrangement for automatic control of the voltage of an electrical filter, said circuit arrangement comprising a control circuit coupled to an electrical filter for changing the voltage of said filter;

a source of voltage coupled to said control circuit;

a control system having an input coupled to said source of voltage, input means and an output connected to said control circuit for supplying to said control circuit a control magnitude depending upon a control voltage supplied to the input means of said control system; and

a control unit having an input coupled to said source of voltage and output means coupled to the input means of said control system, said control unit comprising a source of DC voltage, a guide capacitor, a charging circuit connecting said guide capacitor to said source of DC voltage whereby the control voltage supplied to said control system is produced by said control unit in accordance with the voltage of said guide capacitor, a discharging circuit connnected in parallel with said guide capacitor and including a controllable resistor having a resistance value dependent upon the voltage of said filter, and cutofi circuit means for supplying a cutoff signal to the input means of said control system, said control system controlling said control circuit for interrupting the filter current without delay when a sparkover occurs in said filter.

7. A circuit arrangement as claimed in claim 6, wherein said control circuit comprises thyristor means and the control magnitude supplied by said control system controls said thyristor means, and wherein the cutoff circuit means of said control unit switches the thyristor means of said control circuit via said control system when a sparkover occurs in said filter.

8. A circuit arrangement as claimed in claim 7, wherein the cutoff circuit means of said control unit comprises a first transistor having an emitter-collector path and a control path, said cutoff signal being derived from a point in the emitter-collector path of said first transistor, a second transistor of complementary type to said first transistor having a control path connected in parallel with the control path of said first transistor, the controllable resistor of said discharging circuit comprising the control path of said second transistor, a first voltage divider having a tap point, each of said first and second transistors having a base electrode connected to said first voltage divider, said first transistor being in its fully conductive condition during normal operation of said filter, a third transistor of the same conductivity type as said first transistor, said third transistor being in its nonconductive condition during normal operation of said filter and switching to its conductive condition upon the occurrence of a sparkover in said filter, said third transistor having an emitter electrode coupled to the emitter electrode of said second transistor, a capacitor, a resistor, said third transistor having a collector electrode coupled to the base electrode of said second transistor via said capacitor and said resistor, the collector electrode of said third transistor being connected to the tap point of said first voltage divider, said capacitor discharging via the control path of said second transistor when said third transistor is in its conductive condition whereby the discharging of said capacitor switches said second capacitor to its conductive condition, a reversing switch, a diode and a second voltage divider connected to the base electrode of said second transistor via said diode, said diode being connected with a polarity which is such that said second voltage divider is supplied a fully advanced control current via said reversing switch in an operating position of said reversing switch and said second transistor is in its nonconductive condition.

9. A circuit arrangement as claimed in claim 8, wherein said control unit further comprises a fourth transistor functioning with said third transistor as a monostable multivibrator, said capacitor and said discharging circuit having time constants which prevent the termination of the fully conductive condition of said second transistor prior to the tennination of the reset condition of said monostable multivibrator.

all"

10. A circuit arrangement as claimed in claim 8 wherein said control unit further comprises a diode connecting said second voltage divider to said reversing switch, a fifth transistor having an emitter-collector path connected to said second voltage divider via said diode, said reversing switch having an operable switching position wherein it connects said second voltage divider to said source of DC voltage, and wherein said circuit arrangement further comprises a current transformer connected between said source of voltage and said control circuit and having an output connected to the input' of said control unit for supplying a signal to said control unit when the current flowing through said filter exceeds a predetermined critical value, the fifth transistor of said control unit being fully controlled by the signal supplied by said current transformer to said control unit.

11. A circuit arrangement as claimed in claim 10, wherein said control unit further comprises a resistor, a second guide capacitor connected to said first-mentioned guide capacitor via said resistor, said control voltage being derived from said second guide capacitor, another resistor, the controllable resistor of said discharging circuit being connected in parallel with said second guide capacitor via said other resistor, the period of interruption of the filter current and the time constant of said second guide capacitor and said discharging circuit being adjusted to each other in a manner whereby upon the termination of the interruption said second guide capacitor is substantially completely discharged, said second guide capacitor and said resistor having a time constant which is such that the voltage of said filter increases as rapidly as possible without overshooting. 

1. A circuit arrangement for automatic control of the voltage of an electrical filter, said circuit arrangement comprising a control circuit coupled to an electrical filter for changing the voltage of said filter; a source of voltage coupled to said control circuit; a control system having an input coupled to said source of voltage, input means and an output connected to said control circuit for supplying to said control circuit a control magnitude, said control magnitude depending upon a control voltage supplied to the input means of said control system; a glow lamp; a voltage divider connecting said glow lamp in parallel with said filter; and a control unit having an input coupled to said source of voltage and output means coupled t the input means of said control system, said control unit comprising a source of DC voltage, a guide capacitor, a charging circuit connecting said guide capacitor to said source of DC voltage whereby the control voltage supplied to said control system is produced by said control unit in accordance with voltage of said guide capacitor, a discharging circuit connected in parallel with said guide capacitor and including a controllable resistor having a resistance value dependent upon the voltage of said filter, a photoelement having a photosensitive area provided in operative proximity with said glow lamp in a manner whereby light produced by said glow lamp impinges upon the photosensitive area of said photoelement, and circuit means coupling said photoelement to said discharging circuit for supplying a control signal dependent upon the irradiation of the photosensitive area of said photoelement whereby said control signal controls the controllable resistor of said discharging circuit to a low resistance value when said glow lamp fails to glow and controls said controllable resistor to a high resistance value when said glow lamp glows.
 2. A circuit arrangement as claimed in claim 1, wherein said control unit further comprises a reversing switch, a switching component having a switching condition which depends upon the switching position of said reversing switch and the magnitude of current flowing through said filter, said switching component being in its conductive condition only during the time between the energization of said circuit arrangement, with the assistance of said reversing switch, and the time during which the current flowing through said filter reaches a critical value for the first time after the energization of said circuit arrangement by said reversing switch, said switching component being connected in a manner whereby said control signal is ineffective when said switching component is in its nonconductive condition.
 3. A circuit arrangement as claimed in claim 2, wherein said control unit further comprises auxiliary circuit means connecting said switching component in parallel to the charging circuit of said guide capacitor.
 4. A circuit arrangement as claimed in claim 2, wherein said control unit further comprises a capacitor, a resistor, a monostable multivibrator having a pair of input and output transistors of the same conductivity type and the controllable resistor of said control unit comprises a first transistor having a base-emitter path coupled via said capacitor and said resistor to the output of said monostable multivibrator whereby when said glow lamp glows, the input transistor of said monostable multivibrator is in its fully conductive condition, a resistor, a decoupling diode, a second transistor, the output of said monostable multivibrator being connected to the negative polarity terminal of said source of DC voltage via said resistor, decoupling diode and the emitter-collector path of said second transistor, said switching component comprising said second transistor whereby when said second transistor is in its fully conductive condition approximately the same potential is applied at the output of said monostable multivibrator as at the extinguished glow lamp, a third transistor having a control path connected in parallel with said first transistor and a base electrode of the third transistor, a reversible voltage divider, a diode connects the base electrode to said reversible voltage divider and conducting the control current of said third transistor, the control current of said third transistor flowing through said reversible voltage divider as long as the current flowing through said filter is below the critical value and said first transistor is in a condition other than its fully conductive condition.
 5. A circuit arrangement as claimed in claim 4, wherein said control unit further comprises a fourth transistor of the same conductivity type as the pair of transistors of the monostable multivibrator, said fourth transistor having a collector electrode, a relay having an energizing winding connected to the collector electrode of said fourth transistor, a control circuit for said relay winding including the base-emitter path of said fourth transistor, said base-emitter path being connected to said relay winding, a decoupling diode connecting the control circuit of said relay in parallel with the output of said monostable multivibrator in a manner whereby when the glow lamp glows and said second transistor is in its nonconductive condition a control current flows, a resistor and a decoupling diode, said fourth transistor having an emitter electrode connected to the collector electrode of said second transistor, the emitter electrode of said fourth transistor having a potential of a magnitude which is such that when said second transistor is in its conductive condition said fourth transistor remains in its fully conductive condition.
 6. A circuit arrangement for automatic control of the voltage of an electrical filter, said circuit arrangement comprising a control circuit coupled to an electrical filter for changing the voltage of said filter; a sourCe of voltage coupled to said control circuit; a control system having an input coupled to said source of voltage, input means and an output connected to said control circuit for supplying to said control circuit a control magnitude depending upon a control voltage supplied to the input means of said control system; and a control unit having an input coupled to said source of voltage and output means coupled to the input means of said control system, said control unit comprising a source of DC voltage, a guide capacitor, a charging circuit connecting said guide capacitor to said source of DC voltage whereby the control voltage supplied to said control system is produced by said control unit in accordance with the voltage of said guide capacitor, a discharging circuit connnected in parallel with said guide capacitor and including a controllable resistor having a resistance value dependent upon the voltage of said filter, and cutoff circuit means for supplying a cutoff signal to the input means of said control system, said control system controlling said control circuit for interrupting the filter current without delay when a sparkover occurs in said filter.
 7. A circuit arrangement as claimed in claim 6, wherein said control circuit comprises thyristor means and the control magnitude supplied by said control system controls said thyristor means, and wherein the cutoff circuit means of said control unit switches the thyristor means of said control circuit via said control system when a sparkover occurs in said filter.
 8. A circuit arrangement as claimed in claim 7, wherein the cutoff circuit means of said control unit comprises a first transistor having an emitter-collector path and a control path, said cutoff signal being derived from a point in the emitter-collector path of said first transistor, a second transistor of complementary type to said first transistor having a control path connected in parallel with the control path of said first transistor, the controllable resistor of said discharging circuit comprising the control path of said second transistor, a first voltage divider having a tap point, each of said first and second transistors having a base electrode connected to said first voltage divider, said first transistor being in its fully conductive condition during normal operation of said filter, a third transistor of the same conductivity type as said first transistor, said third transistor being in its nonconductive condition during normal operation of said filter and switching to its conductive condition upon the occurrence of a sparkover in said filter, said third transistor having an emitter electrode coupled to the emitter electrode of said second transistor, a capacitor, a resistor, said third transistor having a collector electrode coupled to the base electrode of said second transistor via said capacitor and said resistor, the collector electrode of said third transistor being connected to the tap point of said first voltage divider, said capacitor discharging via the control path of said second transistor when said third transistor is in its conductive condition whereby the discharging of said capacitor switches said second capacitor to its conductive condition, a reversing switch, a diode and a second voltage divider connected to the base electrode of said second transistor via said diode, said diode being connected with a polarity which is such that said second voltage divider is supplied a fully advanced control current via said reversing switch in an operating position of said reversing switch and said second transistor is in its nonconductive condition.
 9. A circuit arrangement as claimed in claim 8, wherein said control unit further comprises a fourth transistor functioning with said third transistor as a monostable multivibrator, said capacitor and said discharging circuit having time constants which prevent the termination of the fully conductive condition of said second transistor prior to the termination of the reset condItion of said monostable multivibrator.
 10. A circuit arrangement as claimed in claim 8, wherein said control unit further comprises a diode connecting said second voltage divider to said reversing switch, a fifth transistor having an emitter-collector path connected to said second voltage divider via said diode, said reversing switch having an operable switching position wherein it connects said second voltage divider to said source of DC voltage, and wherein said circuit arrangement further comprises a current transformer connected between said source of voltage and said control circuit and having an output connected to the input of said control unit for supplying a signal to said control unit when the current flowing through said filter exceeds a predetermined critical value, the fifth transistor of said control unit being fully controlled by the signal supplied by said current transformer to said control unit.
 11. A circuit arrangement as claimed in claim 10, wherein said control unit further comprises a resistor, a second guide capacitor connected to said first-mentioned guide capacitor via said resistor, said control voltage being derived from said second guide capacitor, another resistor, the controllable resistor of said discharging circuit being connected in parallel with said second guide capacitor via said other resistor, the period of interruption of the filter current and the time constant of said second guide capacitor and said discharging circuit being adjusted to each other in a manner whereby upon the termination of the interruption said second guide capacitor is substantially completely discharged, said second guide capacitor and said resistor having a time constant which is such that the voltage of said filter increases as rapidly as possible without overshooting. 